Semiconductor devices have been known that have an insulation substrate made of, for example, aluminum nitride, front and back metal plates made of pure aluminum, a semiconductor element joined to the front metal plate by, for example, soldering, a heat sink serving as a heat radiating device joined to the back metal plate. The metal plates are joined to each of the front and back surfaces of the insulation substrate. The heat sink is coupled to the back metal plate to be thermally conductive with the back metal plate. The heat sink radiates heat generated by the semiconductor element. The above described semiconductor devices are required to maintain heat radiating performance of the heat sink for an extended period of time. However, depending on the use conditions, thermal stress is generated by the difference in coefficient of linear expansion between the insulation substrate, the metal plates, and the heat sink of the conventional configuration. This can cause joint portions to crack and warp, lowering the heat radiating performance of the heat sink.
To eliminate such a drawback, Japanese Laid-Open Patent Publication No. 2003-17627 discloses a semiconductor module having thermal stress relaxation portions on the back metal plate. The thermal stress relaxation portions are formed by steps, grooves, or recesses that have a predetermined depth. The number and the size of the thermal stress relaxation portions are determined such that the volume ratio of the back metal plate to the front metal plate is not more than 0.6.
Japanese Laid-Open Patent Publication No. 2007-173405 discloses a semiconductor module in which, on a joint surface of the back metal plate with the heat sink, a non-joint region formed by holes or grooves and a joint region where no holes or grooves are formed. The area of the joint region is set to be 65% to 85% of the entire joint surface of the back metal plate.
In the semiconductor module disclosed in Japanese Laid-Open Patent Publication No. 2003-17627, the steps, grooves, or recesses, which serve as thermal stress relaxation portions formed on the back metal plate, relax thermal stress generated in the semiconductor module when the temperature changes. Thus, in order to increase the thermal stress relaxing performance, the steps, grooves, or recesses are preferably as large as possible. However, increasing the size of the steps, grooves, or recesses, in turn, reduces the joint area between the back metal plate and the heat sink. This reduces the thermal conductivity of the back metal plate. Thus, the balance between the thermal conductivity and the thermal stress relaxing performance needs to taken into consideration. That is, the larger the steps, grooves, or the recesses forming thermal stress relaxation portions, the lower the heat radiation efficiency becomes. Thus, there is a limit to improvement of the stress relaxation performance.
Likewise, according to the semiconductor module disclosed in Japanese Laid-Open Patent Publication No. 2007-173405, the larger the non-joint region, the lower the thermal conductivity of the back metal plate of the semiconductor module becomes. This puts a limit on improvement of the stress relaxation performance.
Particularly, in the case of a semiconductor device such as a power module, on which a semiconductor element generating a great amount of heat is mounted, there is a demand for improving the function to relax the thermal stress generated in the semiconductor device without lowering the heat radiation efficiency. Japanese Laid-Open Patent Publication No. 2004-6717 discloses a power semiconductor device that includes an insulation substrate, front and back metal plates (low thermal expansion coefficient metal plates) joined to each of the front and back surfaces of the insulation substrate, a power semiconductor element joined to the front surface of the front metal plate by, for example, soldering, and a heat sink coupled to the back metal plate to be thermally conductive with the back metal plate. The back metal plate has a linear expansion coefficient that is of the same order of the linear expansion coefficients of the power semiconductor element and the insulation substrate. The heat sink has a plurality of partitioning walls defined by a plurality of grooves formed in the heat sink. The partitioning walls are arranged in regions that correspond to the insulation substrate. The distal end of each partitioning wall is not fixed. Thus, the rigidity of the heat sink of the power semiconductor device disclosed in the publication is lower than that of a heat sink in which the distal ends of partitioning walls are fixed. Therefore, thermal stress generated in the heat sink and the insulation substrate is reduced by deformation of the heat sink. However, since the partitioning walls are arranged only in a region that corresponds to the insulation substrate through the low thermal expansion coefficient metal plates, the rigidity of the heat sink cannot be made sufficiently low. The heat sink therefore cannot sufficiently reduce thermal stress. Also, Japanese Laid-Open Patent Publication No. 2004-6717 discloses a structure in which partition walls with free distal ends are provided below a region to which the low thermal expansion coefficient metal plates are not joined. However, since the partitioning walls with free distal ends reduce the rigidity of the heat sink, the rigidity of the heat sink, which has a greater widthwise length than that of the low thermal expansion coefficient metal plates, may be lowered below the minimum rigidity required for the heat sink.
Further, Japanese Laid-Open Patent Publication No. 5-299549 discloses a heat transfer cooling device that includes a box and a plurality of partitioning walls. The partitioning walls define a plurality of flow passages in the box. The partitioning walls are arranged along the diagonals of the base of the box, such that the space between adjacent partitioning walls is reduced toward the diagonals. In the heat transfer cooling device disclosed in Japanese Laid-Open Patent Publication No. 5-299549, spaces between adjacent partitioning walls are smaller in a central part of the heat transfer cooling device, where the temperature easily rises, so that the number of the partitioning walls is increased in the central part. Thus, the heat radiation efficiency at the central part of the cooling device is higher than that in the other parts.
However, in the heat transfer cooling device of Japanese Laid-Open Patent Publication No. 5-299549, since a plurality of partitioning walls are arranged from one corner of the cooling device toward another corner at predetermined intervals, the rigidity of the cooling device is increased. Therefore, when the temperature of the heat transfer cooling device changes, the device cannot exert sufficient stress relaxation performance.
Accordingly, it is an objective of the present invention to provide a semiconductor device that is excellent in heat radiating performance and reliably relaxes stress. Another objective of the present invention is to provide a semiconductor device that prevents the rigidity of a heat sink from being lowered.
To achieve the foregoing objective and in accordance with a first aspect of the present invention, a semiconductor device including an insulation substrate, a metal wiring layer, a semiconductor element, a heat sink, and a stress relaxation member is provided. The insulation substrate has a first surface and a second surface that is opposite to the first surface. The metal wiring layer is joined to the first surface of the insulation substrate. The semiconductor element is joined to the metal wiring layer. The heat sink is arranged on the second surface of the insulation substrate. The stress relaxation member is made of a material having a high thermal conductivity. The stress relaxation member is located between the insulation substrate and the heat sink in such manner as to couple the insulation substrate and the heat sink such that heat can be conducted therebetween. The heat sink has a plurality of partitioning walls that extend in one direction and are arranged at intervals. The stress relaxation member has a stress absorbing portion that is formed by a hole. The hole either extends through the entire thickness of the stress relaxation member or opens in one of both surfaces in the direction of the thickness. The hole is formed such that its dimension along the longitudinal direction of the partitioning walls is greater than its dimension along the arranging direction of the partitioning walls.
In accordance with a second aspect of the present invention, a semiconductor device including an insulation substrate, a metal wiring layer, a semiconductor element, a heat sink, and a stress relaxation member is provided. The insulation substrate has a first surface and a second surface that is opposite to the first surface. The metal wiring layer is joined to the first surface of the insulation substrate. The semiconductor element is joined to the metal wiring layer. The heat sink is arranged on the second surface of the insulation substrate. The stress relaxation member is made of a material having a high thermal conductivity. The stress relaxation member is located between the insulation substrate and the heat sink in such manner as to couple the insulation substrate and the heat sink such that heat can be conducted therebetween. The heat sink has a plurality of partitioning walls that extend in one direction and are arranged at intervals. The stress relaxation member has a stress absorbing portion. The stress absorbing portion includes a plurality of groups of through holes extending through the entire thickness of the stress relaxation member. The through holes are arranged along the longitudinal direction of the partitioning walls. Each of all the through holes is formed such that its opening dimension along the arranging direction of the partitioning walls is greater than its opening dimension along the longitudinal direction of the partitioning walls. In each of the groups of through holes, the sum of the opening dimensions of the through holes along the longitudinal direction of the partitioning walls is longer than the maximum width of the stress absorbing portion along the arranging direction of the partitioning walls.
In accordance with a third aspect of the present invention, a semiconductor device including an insulation substrate, a first metal plate, a semiconductor element, a second metal plate, and a heat sink is provided. The insulation substrate has a first surface and a second surface that is opposite to the first surface. The first metal plate is joined to the first surface of the insulation substrate. The semiconductor element is joined to the first metal plate. The second metal plate is joined to the second surface of the insulation substrate. The heat sink cools the semiconductor element, and is coupled to the second metal plate such that heat can be conducted. The heat sink includes a case portion and a plurality of partitioning walls located in the case portion. The partitioning walls define a plurality of cooling medium passages. The case portion has a surface that faces the second metal plate, which surface includes a joint region, to which the second metal plate is joined, and a non-joint region, to which the second metal plate is not joined. Each partitioning wall includes a first end facing the second metal plate and a second end opposite to the first end. The partitioning walls include first partitioning walls and second partitioning walls. The first end of each first partitioning wall is joined to an inner surface of the case portion. The second end of each first partitioning wall is not joined to an inner surface of the case portion. The first and second ends of each second partitioning wall are joined to inner surfaces of the case portion. Among the first and second partitioning walls, at least one or more of the first partitioning walls pass through a region in the case portion that corresponds to the joint region. Among the first and second partitioning walls, only one or more of the second partitioning walls pass through a region in the case portion that corresponds to the non-joint region.
In accordance with a fourth aspect of the present invention, a semiconductor device including an insulation substrate, a first metal plate, a semiconductor element, a second metal plate, and a heat sink is provided. The insulation substrate has a first surface and a second surface that is opposite to the first surface. The first metal plate is joined to the first surface of the insulation substrate. The semiconductor element is joined to the first metal plate. The second metal plate is joined to the second surface of the insulation substrate. The heat sink cools the semiconductor element, and is coupled to the second metal plate such that heat can be conducted. The heat sink includes a case portion and a plurality of partitioning walls located in the case portion. The partitioning walls define a plurality of cooling medium passages. All the partitioning walls are located in a region in the case portion that is directly below the semiconductor element.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.